Piezoelectric motor control

ABSTRACT

A piezoelectric system has a piezoelectric motor ( 20 ) driving a driven element ( 22 ) so as to move the driven element ( 822 ) in response to an electric signal ( 25 ). The motor ( 20 ) has at least a first optimal operating frequency at which the motor ( 20 ) moves the driven element ( 22 ) an amount that meets predetermined criteria. The motor ( 20 ) and driven element ( 22 ) have a desired performance criteria when operated at that first operating frequency. A plurality of concatenated sinusoidal sweeping frequencies are repeatedly supplied to the piezoelectric motor ( 20 ) with at least one of the sweeping frequencies being sufficiently close to the first operating frequency to cause detectable motion of the driven element ( 22 ). The frequencies are varied in response to movement of at least one of the motor ( 20 ) and the driven element ( 22 ) to produce an average performance of the motor ( 20 ) and driven element ( 22 ) for a time corresponding to the time for one sweep of frequencies. The average performance is greater than an actual performance of the driven element ( 22 ) for the same period of time but when the actual performance is less than the desired performance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/355,172, filed Feb. 6, 2002, the entire content of which is herebyincorporated by reference.

FIELD OF INVENTION

This invention relates to a method and apparatus for controllingpiezoelectric motors and the resulting combination of piezoelectricmotors and the control systems therefor.

BACKGROUND OF THE INVENTION

In piezoelectric motors, one or more piezoelectric elements are excitedwith electrical signals to extend and contract in order to generate amicroscopic mechanical motion within the motor that is transformed intoa macroscopic motion of a driven element. In part, piezoelectric motordesigns differ in the electric signals used to excite the motions, inthe form of the microscopic motion, and in the mechanism used totransform the microscopic motion into a macroscopic motion.

Piezoelectric motors take various forms and have various controlsystems. Some piezoelectric motors operate principally with sinusoidalelectric signals of a single frequency, and are referred to here assingle-frequency motors. The single frequency piezoelectric motorscontrast with piezoelectric motors that require special waveforms foroperation, such as triangular waveforms, such shaped waveforms havingfrequency spectra that are the composite of many frequencies with theoverall result being a shaped waveform. Some single-frequencypiezoelectric motors may also be operated with electrical signals thatcontain other frequency components, but it is not necessary to includeadditional frequency components for proper operation of a singlefrequency piezoelectric motor. Single-frequency piezoelectric motors mayalso have more than one operational frequency that, when used atdistinct times, result in distinct macroscopic motions of the drivenelement at those times. For example, U.S. Patent Publication No.2002/0038987A1, the entire contents of which are hereby incorporated byreference, discloses embodiments that include single-frequencypiezoelectric motors that have two distinct operational frequencies, onefor a forward motion and one for a backward motion of a driven element.

The optimal frequency of operation, i.e., the frequency at which themechanical output and performance of a piezoelectric motor is in somesense optimal, is typically related to a mechanical resonance. Theoptimal frequency therefore varies with several factors, such astemperature. Ambient temperature can change and vary the performance,and piezoelectric motors warm up during operation and that can affectperformance. Further effects that influence the optimal frequency of apiezoelectric motor during its lifetime include fatigue, wear such asabrasion between the piezoelectric motor and the driven element, andother factors. Furthermore, differences during manufacturing andassembly and general tolerances result in a different optimal frequencyfor any two piezoelectric motors of the same design and manufacture.Finally, even if the optimal operating frequency was known beforehand,it is not guaranteed that the electronic circuit supplying the electricsignal is able to generate the optimal frequency exactly, since thecircuitry itself is subject to effects of temperature changes, aging,and manufacturing tolerances.

There is thus a need for an electrical driving circuit that drives apiezoelectric motor at or near its optimal frequency of operation byemploying means of control. Prior art includes Phase Locked Loop (PLL)feedback control solutions. It is known that when a typicalpiezoelectric motor is excited close to its operational resonancefrequency, there occurs a phase difference between the excitation signaland the vibration of the piezoelectric motor. If the vibration can bemeasured, a PLL may be able to exploit this phase difference andcontinuously track the operation frequency of the piezoelectric motor.PLL requires a dedicated continuously operating control circuit, and itis limited by the frequency range in which a phase difference isdiscernible, and is further limited by various electrical noise factors.PLL works only for piezoelectric motors where there is a clearmonotonous relationship between the measured phase difference and thequality (strength, speed, etc.) of the resulting macroscopic motion.This relationship may not exist for all piezoelectric motor designs.

There is thus a need for control schemes that can drive asingle-frequency piezoelectric motor sufficiently near its optimalfrequency of operation but that are less dependent on theparticularities of the piezoelectric motor and that can accommodate morevariation in the piezoelectric motor design and manufacture.

BRIEF SUMMARY OF THE INVENTION

A piezoelectric motor is provided that is in driving contact with adriven element so as to move the driven element in response to anelectric signal provided to the motor. The motor has at least a firstoptimal operating frequency at which the motor moves the driven elementan amount that meets predetermined operational criteria. The motor anddriven element have a desired performance criteria when operated at thatfirst operating frequency. As the motor and/or the driven elementdegrade, or as manufacturing tolerances cause the motor and drivenelement to perform less efficiently than desired, or as the signal tothe motor varies from the optimal driving frequency, the performancebegins to degrade outside the desirable limits, and ultimately degradesto a point where the performance is outside an acceptable range ofperformance criteria.

To compensate for this natural performance degradation, a plurality ofconcatenated sweeping frequencies is repeatedly supplied to thepiezoelectric motor. At least one of the sweeping frequencies issufficiently close to the first operating frequency or to an alternativeresonance frequency of the motor and/or combined motor and drivenelement to cause detectable motion of the driven element. Preferably,the sweeping frequencies result in an average performance that exceedsthe performance of the motor and/or driven element when they begin todeviate from the desired performance criteria. The composition of thesweeping frequencies can be varied in order to maximize the performanceso that it approaches, and preferably closely approximates or achievesthe desired performance criteria.

The detectable motion is preferably used to vary the sweepingfrequencies in order to help optimize an average performance over theperiod of time it takes for the sweeping frequencies to complete onecycle. The detectable motion is also preferably used to help selectwhich frequencies to use in the sweeping frequencies. The composition ofthe sweeping frequencies can be varied on a periodic basis, or usingpredetermined criteria in order to help optimize the averageperformance. Thus, the frequencies in the plurality of concatenatedsweeping frequencies are preferably varied in response to movement of atleast one of the motor and the driven element to produce an averageperformance of the motor and driven element for a time corresponding tothe time for one sweep of frequencies, is greater than an actualperformance of the driven element for the same period of time but whenthe actual performance is less than the desired performance.

Preferably, the desired performance criteria includes at least one ofthe speed of the driven element or motor, the force exerted by the motoron the driven element, the force exerted by the driven element, and thepower consumed by the motor. The plurality of concatenated sweepingfrequencies can be a continually increasing series of frequencies, acontinually decreasing series of frequencies, or a variety offrequencies. The selected frequencies used to achieve a maximumperformance will vary with the particular application, but arepreferably sufficiently close to a resonant mode of the motor, drivenelement, or the combined motor and driven element so that the averageperformance is maximized and approximates the desired performancecriteria. The combined period of the swept frequencies and the period ofthe individual frequencies within the swept frequencies can be varied toapproximate or achieve the desired performance criteria.

Preferably, but optionally, the sweeping frequencies are variedperiodically or according to some other criteria in order to maintainthe average performance criteria at its desired value, and the desiredvalue is usually as close to possible to the desired performancecriteria. When the sweeping frequencies are varied, the varied sweepingfrequencies preferably include at least one frequency which causessufficient motion of one of the motor or driven element to be detectedby a sensor, and feedback from that sensor can help to optimize theaverage performance as well help identify which frequencies may be bestincluded within the sweeping frequencies.

There is also advantageously provided a method for controlling apiezoelectric motor in which the piezoelectric motor is configured tomove a driven element when a sinusoidal electric signal of a firstfrequency is supplied to the piezoelectric motor with an amplitude thatis sufficient to move a driven element a predetermined distance. Themethod includes selecting a predetermined first sequence of frequencies,where the first sequence of frequencies comprises at least two mutuallydifferent sinusoidal frequencies. Individual waveforms are createdcorresponding to each frequency of the first sequence of frequencies sothat each individual waveform has a predetermined finite duration andamplitude and is periodic with a period that is the inverse of thecorresponding frequency. The individual waveforms are concatenated intoa single first electric signal and that first signal is suppliedrepeatedly to the piezoelectric motor to move the driven element. Theselected first sequence of frequencies includes a sufficient number offrequencies that are distributed to cause the piezoelectric motor tomove the driven element even when the properties of the piezoelectricmotor change within a predictable range.

Thus, as the motor, driven element, signal source, or other componentscause the performance to deviate from the desired performance criteria,the first sequence of frequencies provides a performance that, whenaveraged over the duration of the single first electric signal,preferably, but optionally, does not vary more than 30% when theproperties of the piezoelectric motor change. Preferably, butoptionally, the single first electric signal causes the piezoelectricmotor to move the driven element with a varying performance.

Moreover, the method further advantageously, but optionally includesmonitoring the motion of the driven element. The first sequence offrequencies is preferably further selected to include a sufficientnumber of frequencies that are distributed to cause the piezoelectricmotor to move the driven element so that the motion of the drivenelement is maintained within a predetermined value as determined by themonitoring of the driven element. The monitoring advantageously, butoptionally, uses a motion detector that provides a feedback signal whenthe motion of the driven element passes at least one selected threshold.

The feedback signal can be analyzed to determine an estimated frequencyat which the piezoelectric motor can move the driven element when asinusoidal electric signal of the estimated frequency is supplied to thepiezoelectric motor. A second sequence of frequencies can be selectedthat preferably comprises at least the estimated frequency and one otherfrequency that is different from the estimated frequency to generate asecond electric signal in accordance with the method of generating thefirst electric signal to cause the piezoelectric motor to move thedriven element with an average performance that is higher than theaverage performance of the piezoelectric motor was before the estimatedfrequency was determined. The steps of analyzing the feedback signal andselecting a second sequence of frequencies can be repeated as often asneeded to achieve a desired duration and distance of motion and adesired average performance. Advantageously, the second sequence offrequencies comprises at least one frequency that is smaller than theestimated frequency and at least one frequency that is larger than theestimated frequency.

The method also advantageously comprises analyzing the feedback signalto determine if the motion of the driven element has been less than apredetermined value for a predetermined amount of time. The secondsequence of frequencies can be modified when the predetermined amount oftime has passed so there is at least a difference between the largestand the smallest frequency of the sequence that is larger than thedifference between the largest and the smallest frequency of theunmodified second sequence. These steps of analyzing the feedback signaland modifying the second sequence—until it is determined that the motionof the driven element is no longer less than the predetermined value forthe predetermined amount of time—can be repeated as needed, preferablyuntil the desired performance criteria is approximated as closely as ispossible.

The above method preferably selects the first sequence of frequencies tocause the piezoelectric motor to move the driven element by a defineddistance even if the properties of the piezoelectric motor change due topredictable causes. The method can further include supplying theelectric signal a predetermined number of times per second in order tocause the piezoelectric motor to move the driven element at a definedspeed. Moreover, any two consecutive frequencies advantageously eachproduce a piezoelectric motor performance comprising at least one of(the speed of the driven element, the motion of the driven element, andpower consumption of the motor), with a performance difference betweeneach of two said consecutive frequencies that is no more than apredetermined value.

In some embodiments, the frequencies are selected to cause thepiezoelectric motor to produce a predetermined audible sound. This couldhave a variety of applications in various types of toys andentertainment applications.

In a further embodiment, the method includes selecting at least twosinusoidal frequencies that are mutually different to form a sequence offrequencies to cause the piezoelectric motor to move the driven elementwhen individual waveforms corresponding to each frequency of thesequence of frequencies are concatenated to form an electric signal thatis supplied repeatedly to the piezoelectric motor to move the drivenelement. Each of these individual waveforms has a predetermined finiteduration and amplitude and are periodic with a period that is theinverse of the corresponding frequency. The at least two sinusoidalfrequencies are selected to further cause the piezoelectric motor tomove the driven element even if when the properties of the piezoelectricmotor change within a predictable range. Deviating from the desiredperformance criteria a predetermined amount would be such a change.

In further variations of this further embodiment, the at least twofrequencies are selected to cause the piezoelectric motor to move thedriven element by a defined distance. Moreover, the duration of each ofthe individual waveforms can be selected to cause the piezoelectricmotor to move the driven element with a defined speed. Still further,the at least two frequencies can each cause the piezoelectric motor tomove the driven element with a different performance.

As with the above embodiments, this further embodiment can includemonitoring the motion of the driven element and selecting the sequenceof the at least two frequencies to further include a sufficient numberof frequencies that are distributed to cause the piezoelectric motor tomove the driven element so that the monitored motion of the drivenelement meets a predetermined criteria. Preferably, this furtherembodiment of the piezoelectric motor includes a motion detectorproviding a feedback signal when the motion of the driven element passesat least one selected threshold. The feedback signal can be analyzed todetermine an estimated frequency at which the piezoelectric motor canmove the driven element when a sinusoidal electric signal of theestimated frequency is supplied to the piezoelectric motor. The at leasttwo frequencies are preferably selected to comprise the estimatedfrequency to cause the piezoelectric motor to move the driven elementwith an average performance that is higher than an average performanceof the piezoelectric motor before the estimated frequency wasdetermined.

Preferably, the piezoelectric motor of this further embodiment includesa motion detector providing a feedback signal when the motion of thedriven element passes a selected threshold. Again, the feedback signalcan be analyzed to determine if the motion of the driven element hasbeen less than a predetermined movement for a predetermined amount oftime. The sequence of the at least two frequencies can be modified whenthe predetermined amount of time has passed in order to cause at least adifference between the largest and the smallest frequency of thesequence that is larger than the difference between the largest and thesmallest frequency of the unmodified sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

These as well as other features of the present invention will becomemore apparent upon reference to the drawings, in which like numbersrefer to like parts throughout, and in which:

FIG. 1 is a graph showing piezoelectric motor performance versusfrequency to illustrates the influence of sequences of frequencies onpiezoelectric motor performance;

FIG. 2 illustrates typical performance curves obtained by feedback;

FIG. 3 illustrates a feedback control method;

FIG. 4 is a block conceptual diagram with a feedback loop for a singlefrequency piezoelectric motor that is in driving communication with adriven element;

FIG. 5 shows several digital signals;

FIG. 6 is a block diagram with multiple single-frequency piezoelectricmotors that are in driving communication with the same driven element.

DETAILED DESCRIPTION

Referring to FIG. 4, a piezoelectric motor 20 is provided and configuredto be in driving contact with a driven element 22. The piezoelectricmotor 20 is of the type that can be controlled to produce usefulmacroscopic motion of the driven element 22 by applying to thepiezoelectric motor 20 a single electric signal 25 that is sinusoidal ofa certain frequency. The term sinusoidal as used herein includeswaveforms that are phase shifted, such as cosine waves. The range offrequencies for which useful motion is produced will be referred toherein as the range of operation. The range of operation is typically acoherent interval of frequencies within which the macroscopic motion ofthe driven element 22 occurs in the same direction. For thepiezoelectric motors of the type discussed herein, the motion of thedriven element 22 is understood to be a macroscopic motion of the drivenelement 22 that is the composite of a multitude of small displacementsof the driven element 22, the small displacements being caused by thepiezoelectric motor 20 and being substantially in the same direction.The bold arrow on the driven element 22 in FIG. 4 indicates a possibledirection, but the opposing direction may also be possible. The drivenelement 22 is shown as a wheel, but other driven elements, such as rods,plates and balls may be used that provide further possibilities fordirections of motions. A piezoelectric motor 20 may have severaldisjoint ranges of operation in which the driven element 22 moves indifferent directions. The macroscopic motion of the driven element 22resulting from piezoelectric motor 20 operation is typically optimalwith respect to some performance criterion at an optimal frequencywithin the range of operation. The performance criterion can vary, buttypically includes one or more of the speed of the driven element 22,the force that the piezoelectric motor 20 generates, or a combination ofthe two, but the criteria may also include the electric powerconsumption. A typical piezoelectric motor 20 generally has a betterperformance if the same motion of the driven element 22 is achieved withless electric power consumption. Other performance criteria could apply.The optimal frequency and the boundaries of the range of operation ofthe piezoelectric motor 20 are expected to differ between any twopiezoelectric motors of the same build due to design and manufacturingtolerances, material variations, etc. The piezoelectric motor 20 itself,and the materials used to make the piezoelectric motor 20 and anycontrol circuitry associated therewith, are also expected to change overtime due to wear, warming, aging, etc.

The block diagram in FIG. 4 further shows the piezoelectric motor 20comprising one or more piezoelectric elements 21, which may be of thesingle- or multilayer type, and a mechanically resonating element 28.Further are shown means 24 of generating an electric signal 25 to besupplied to the one or more piezoelectric elements 21, and a controller23 that controls the means 24. A wide variety of frequency generators,drivers and control circuits are known in the art, and a large numberare commercially available for use as the signal generating means 24 andthe controller 23. The controller 23 may or may not be supplied with afeedback signal 27 from a motion detecting device 26 that detects motionof the driven element 22 and/or with a feedback signal 30 that isobtained from the piezoelectric motor 20 or the associated electriccomponents. For the control methods disclosed herein, the controller 23operates principally in an open-loop fashion, but the controller 23 mayintermittently use the feedback signals 27 and/or 30 to adapt itsopen-loop control strategy.

The performance of the piezoelectric motor 20 as measured by a selectedperformance criterion varies within the range of operation. Theperformance typically increases from the boundaries of the range ofoperation towards the optimal frequency. The general shape of aperformance curve of a representative piezoelectric motor 20 at somepoint in time as a function of frequency is indicated in FIG. 1 by curve102. The shape of the performance curve reflects how the performancedepends on the excitation frequency f₁ to f₅ or any particular frequencyf_(n). The shape of the performance curve may also vary over time.

The preferred control methods employ predetermined sequences of non-zerofrequencies for exciting the piezoelectric motor 20. The frequencies ofa particular sequence are selected from a control range of frequencies,where the control range usually includes at least the aforementionedrange of operation, so that the sequence comprises at least onefrequency from the range of operation. In a sequence of frequencies, atleast two frequencies are mutually different, meaning that they have adifferent frequency rather than the same frequency with differentdurations of associated waveforms, said waveforms being explained below.For example, mutually exclusive frequencies would not include two ormore sequential signals each having the same frequency but different fordurations of time. Mutually exclusive frequencies would include twosequential signals each having a frequency that differed from the otherby only a few hertz but were of the same duration, or even of differentdurations.

To generate an electric signal 25 for controlling the piezoelectricmotor 20 using means of controlling 23 from such a sequence offrequencies, first a waveform is created for each frequency of thesequence, each waveform having a predetermined finite duration andamplitude, and each waveform further being periodic with a period thatis the inverse of the corresponding frequency. Said predetermined finiteduration is understood to be at least as long as one period of thecorresponding waveform. The waveforms are then linked together insequence (concatenated) in order to compose the electric signal 25 usingappropriate electronic means 24.

For example, if {f₁, f₂, f₃} is a predetermined sequence of frequencies,then {w₁, w₂, w₃} constitutes the electric signal wherein w₁, w₂, and w₃are periodic waveforms with periods 1/f₁, 1/f₂, 1/f₃, each waveformhaving a duration of T₁, T₂, T₃ and an amplitude A₁, A₂, A₃,respectively. Useful periodic waveforms are the sinusoidal (harmonic),triangular (saw tooth), rectangular (digital) waveform. This list isnon-exhaustive.

Waveforms can be generated by changing the phase of another waveform.For example, a cosine is a sine with a 90-degree phase shift. Asequence, or, equivalently, the corresponding electric signal 25, can berepeated as often as needed. A further example of a sequence that can beused in the proposed method is the periodic frequency sweep. In such asweep, the sequence consists of frequencies that are monotonicallyincreasing, or decreasing, between the two boundaries of the controlrange.

The sequences are preferably predetermined or random but within apredetermined distribution. Appropriate sequences include periodicallyrepeated sweeps from low to high frequencies (up-sweeps) or from high tolow frequencies (down-sweeps), or oscillating sweeps (an up-sweepfollowed by a down-sweep, and so on). When the electric signal 25 issupplied to the piezoelectric motor 20, the average piezoelectric motor20 performance, i.e., the average speed or driving force it isgenerating in the driven element 22, for instance, a combination of thetwo, or the electric power consumption, can be regulated byappropriately choosing the frequency distribution of the correspondingsequence, for example, by selecting a starting and ending frequency fora sweep.

The speed in which a sequence is executed depends on the durations ofthe waveforms of the corresponding electric signal 25. The speed inwhich a sequence can be executed can also be used to further adjustpiezoelectric motor 20 performance and influence acoustical noisegeneration. Acoustical noise in the form of a clicking or similar soundmay occur, for example, when the sequence of frequencies correspondingto the electric signal 25 supplied to the piezoelectric motor 20comprises two frequencies g₁ and g₂, g₂ either immediately following g₁in the sequence or immediately preceding it, g₁ being close to theoptimal frequency and g₂ being in the non-operational range. A frequencyg₁ follows g₂ if g₁ is at the beginning of the sequence and g₂ is at theend of the sequence and vice versa. Depending on how often thetransition from g₁ to g₂, or vice versa, occurs per second, i.e.,depending on the frequency of transitions, different acoustical noisemay be generated.

A frequency of transitions in the order of 2 kHz is believed to create anoise level that is perceived as particularly unpleasant to humanhearing compared to other frequencies given the same sound volume. Byincreasing of decreasing the frequency of transitions, e.g., byincreasing or decreasing the durations of the waveforms that have thefrequencies g₁ and g₂, the noise is not eliminated but can be shifted infrequency to a range that is less disturbing to the human hearing, toanimal hearing, or possibly to sound sensitive equipment. Alternatively,this sort of noise generation can be minimized or eliminated by avoidingor limiting said transitions altogether. This can be achieved byrequiring that any two consecutive frequencies of a sequence produce apiezoelectric motor 20 performance with a performance difference orchange that is no more than a predetermined value as measured by theselected performance criterion when an electric signal 25 that issinusoidal of either of these frequencies is supplied to thepiezoelectric motor 20.

It is possible to further select sequences of frequencies forcontrolling a piezoelectric motor 20 to purposefully generate an audiblesound from the piezoelectric motor 20. In addition, and as a furtherexample, periodic interruptions of the driver signal 25 at appropriatetimes can generate audible noise if the frequency of the interruptionlies in the audible range of living creatures, including humans,animals, fish, reptiles or insects. The audible range is typicallybetween about 20 Hz to about 18,000 Hz for humans, but will vary withage. This intentional generation of driving noise can be used tosimulate engine sounds in toys, or to generate other noises that haveapplication for toys or other uses. The speed and noise of thepiezoelectric motor 20 can further be controlled by modulating theamplitude or the waveform of the driving signal 25. The methods can beused alone or in combination. There is thus provided means for using apiezoelectric motor 20 to generate useful motion and/or audible signalshaving use in specific applications such as toys or other areas whereaudible signals are used.

The duration of a waveform from a corresponding sequence of frequenciesis a very useful design factor. For example, if a sequence repeatedlytoggles between a first nearly optimal frequency and a second frequencythat lies outside the range of operation then the resulting motion ofthe driven element 22 is principally a stop-and-go motion. Thisstop-and-go motion is clearly noticeable if the durations of thewaveforms corresponding to said sequence are very long, e.g., severalseconds. However, the stop-and-go motion may also be present if saiddurations are extremely short, e.g., only a few times the inverse of oneof the said first or the second frequency. This is due to the extremeresponsiveness of piezoelectric motors. In other words, piezoelectricmotors have typically extremely short transients. If the stop-and-gomotion is faster than the human eye can perceive, for exampleapproximately faster than about 25 Hz stop-and-go cycles per second, theresulting motion of the driven element 22 appears smooth to the unaidedhuman eye. Depending on the number of stop-and-go cycles per second, theresulting motion may also appear smooth to the human touch, or smoothwith respect to other measures. In this sense, the term ‘smooth’reflects an average motion quality of the driven element 22.

As used herein, uniformly increasing or decreasing the durations of thewaveforms for a sequence of frequencies will here be referred to asexecuting the sequence faster or slower.

There is thus provided an averaging effect in which the macroscopicmotion of the driven element 22 appears smooth with an averageperformance that is less than the piezoelectric motor 20 performance atthe optimal frequency if a sequence of frequencies is executedsufficiently fast and if it contains sufficiently many frequenciesinside the range of operation.

A motion of the driven element 22 caused by the piezoelectric motor 20is considered smooth if the piezoelectric motor 20 performancefluctuations during the execution of a sequence of frequencies cannot beperceived by means and criteria of observing or monitoring the drivenelement 22 as set forth by a particular application. In other words, amotion of the driven element 22 is considered smooth if said monitoringmeans and criteria cannot tell that there are indeed piezoelectric motor20 performance fluctuations occurring. For example, in a toy it may besufficient for piezoelectric motor 20 performance fluctuations to occurat a rate that is faster than approximately 25 Hz in order for themotion of the driven element 22 to be perceived as being smooth if themonitoring is performed by an average human observer. Other rates mayapply if the motion is supposed to appear smooth to a non-humanobserver, such as a pet animal. Likewise smooth motion could beidentified by a smooth and continuous sound produced by piezoelectricmotor 20 operation as perceived by a human ear, which may differ fromwhat a dog or cat perceives. In still other applications, the smoothnesscriteria may be based on performance criteria monitored by instruments.Thus, the smoothness of the driven element 22 motion could also bemonitored by electric instruments such as optical sensors, motionsdetectors, or other instruments that sense some parameter resulting frommotion of the piezoelectric element 20 or the driven element 22. Forexample, in some applications it may be required that the motion of thedriven element 22 is perceived as being smooth, i.e., free offluctuations, when means of observing the motion of the driven element22 is the human or non-human touch. A motion may also be determined assmooth or sufficiently smooth by indirect means. For example, the drivenelement may itself be connected to other elements or supports that maybe monitored for vibrations or similar to determine a smoothness ofmotion. A sequence of frequencies is thus considered being executedsufficiently fast and containing sufficiently many frequencies insidethe range of operation if the resulting driven element motion 22 isperceived smooth by monitoring means and criteria of observing thedriven element 22 as set forth by the particular application.

The control range of frequencies usually is selected to include afrequency at which the piezoelectric motor 20 produces a responsivemotion or signal with a desired characteristic such as amplitude,frequency, or phase. If the control range of frequencies is selectedsufficiently large, it should always contain the range of operation evenif the range of operation shifts due to predictable and unpredictablevariations in the piezoelectric motor 20 operation attributable tomanufacturing and production tolerances and further attributable toperformance changes and degradation of the piezoelectric motor 20 andassociated electronic components. Furthermore, the range of operationshould also be contained in the control range even if the electroniccircuit 24 that generates the electric signal 25 and supplies it to thepiezoelectric motor 20 is inexact because of changes attributable tomanufacturing and production tolerances and further attributable toperformance changes and degradation of the associated electroniccomponents. Therefore, if a sequence of frequencies is selected so thatthe frequencies of the sequence are sufficiently dense within thecontrol range, then the aforementioned averaging effect can produce apiezoelectric motor performance that varies typically by less than 30%preferably less than 20%, even more preferable by less than 10% andbetter even by less than 5% when the properties of the piezoelectricmotor 20 change within a predictable range. Concurrently, it can also beadvantageous to desire a performance variation that is more than 5%,10%, 20%, or even 30%. A relatively constant performance can thus beachieved without the need of feedback control.

The term ‘sufficiently dense’ as used here is illustrated schematicallywith reference to FIG. 1, which shows performance vs. frequency graphs.The maximal achievable performance on the vertical axis is labeled 1, 1being equal 100%. In FIG. 1, curve 101 illustrates an idealizedpiezoelectric motor 20 performance as the excitation frequency changes.Curve 101 serves as a reference curve for selecting an appropriatesequence of frequencies and is typically measured under standardizedconditions for a representative group of one or more piezoelectricmotors that are representative of an entire group of piezoelectricmotors of the same design and manufacture. Said entire group may, forexample, comprise one day of the production volume of a manufacturingplant producing the piezoelectric motors 20. The curve 101 may becomputed as the average of all performance curves of the representativegroup of piezoelectric motors, or it may be computed as the minimalperformance that each piezoelectric motor of the representative group isable to deliver under said standardized conditions. Other computationmethods for the curve 101 may be useful as well. Curve 102 illustratesthe piezoelectric motor performance curve of a particular piezoelectricmotor 20 from the entire group of piezoelectric motors at a given time.Curve 102 is usually not explicitly measured or known. Due to the alwayspresent piezoelectric motor parameter fluctuations, curves 101 and 102usually do not coincide. Furthermore, curve 102 varies with time. Theoptimal frequency of curve 101 is marked by a vertical line 103 that isfixed. The optimal frequency of curve 102 is labeled with f₀ and ismarked by a vertical line 104, both of which move with the curve 102 asthe curve 102 varies with time.

For illustrative purposes, a first sequence of frequencies, {f₁, f₂, f₃,f₄, f₅}, comprising five equally-spaced frequencies is selected. Thesequence is repeated as often as needed to achieve a desired totalduration of piezoelectric motor 20 operation. In this example, eachfrequency of the first sequence of frequencies is executed with an equalduration. The total piezoelectric motor 20 performance averaged over thetotal duration of piezoelectric motor 20 operation for the firstsequence of frequencies is approximately one fifth of the sum of thevalues of the curve 102 at the five frequencies of the sequence. Saidtotal piezoelectric motor 20 performance varies with the curve 102, i.e.with the location of curve 104 or, equivalently, with the frequency f₀.The varying total piezoelectric motor 20 performance as a function ofthe frequency f₀ is graphed by curve 106. As shown, the performancereaches a maximum value of approximately 0.3, this value being less thanthe maximum value of 1.0 of curve 102. Curve 106 fluctuates between 0.2and 0.3. In other words, if the piezoelectric motor 20 changes itsproperties due to temperature, etc., and the curve 102 consequentlymoves in between the frequencies f₁ and f₅ to a unknown position, thenexecuting the sequence of frequencies {f₁, f₂, f₃, f₄, f₅} ensures thatthe total piezoelectric 20 motor performance remains within the range0.25+/−0.05, i.e., that it remains 0.05/0.25=20% constant.

For the purpose of comparing total piezoelectric 20 motor performances,the same procedure is now repeated for an exemplary second sequence offrequencies, {f₁, f₂, f₃, f₄, f₅, f₆, f₇, f₈, f₉}, comprising nineequally-spaced frequencies. The total piezoelectric motor 20 performanceaveraged over the total duration of piezoelectric motor 20 operation forthe second sequence of frequencies is approximately one ninth of the sumof the values of the curve 102 at the nine frequencies of the sequence.The varying total piezoelectric motor 20 performance as a function ofthe frequency f₀ is shown by curve 105. The variation of curve 105 isapproximately 0.29+10.01, i.e., the curve is constant within0.01/0.29=3.5%. Curves 105 and 106 have the same maxima, but curve 105has a smaller range of variation and a higher mean. The second sequenceof frequencies, which is denser than the first sequence of frequenciesby having more frequencies within the same bandwidth, the bandwidth ofboth the first and the second sequences of frequencies being thedifference (f₅-f₁), is therefore more robust with respect to variationsof the parameters of the piezoelectric motor 20 and associatedcomponents. For example, the second sequence of frequencies would beconsidered sufficiently dense if a piezoelectric motor 20 performancewas required that remains better than, say, 10% constant and that isabove, say, 0.25.

By sweeping the frequencies within the range of operation, an excitationsignal 25 to the piezoelectric motor 20 is provided that preferablyalways encompasses the optimum performance of the piezoelectric motor20. While a portion of the frequencies are less than optimal, the rangeof operation frequencies is advantageously close enough to the frequencycorresponding to the optimum performance of the motor 20 so that theoverall performance of the piezoelectric motor 20 is likely to begreater than if no swept frequency is provided. This occurs because theperformance of a piezoelectric motor is typically very high for a rangeof frequencies on either side of the optimal frequency, and by sweepingthose frequencies an averaged performance is achieved that is likely tobe higher than will occur if a single fixed excitation frequency isselected and provided without prior knowledge of curve 102. Thisincreased performance by sweeping a control range of frequencies alsoallows continued high performance when the piezoelectric motor 20 getshot, ages, or otherwise undergoes a change that causes the optimalfrequency to change. There is thus provided an open-loop control methodthat can produce steady performance of piezoelectric motor 20independently of parameter fluctuations of a piezoelectric motor 20.

The use of sequences of frequencies to drive the piezoelectric motor 20can provide a number of other advantages and uses. The order of thefrequencies in a sequence may be re-arranged to satisfy otherconditions. For example, the aforementioned sequence {f₁ . . . f₉} maybe understood as the composition of two up-sweeps. The same sequencewritten as {f₁, f₆, f₂, f₇, f₃, f₈, f₄, f₉, f₅} is a smooth up-sweepthat has advantageously small frequency steps but one large frequencyjump from f₅ to f₁ in between repetitions. The same sequence written as{f₁, f₂, f₃, f₄, f₅, f₉, f₈, f₇, f₆} is an overall smooth composition ofan up-sweep with a down-sweep, which can be advantageous in applicationswhere frequency jumps may lead to undesired audible noises. In thiscontext, it should be noted that a monotonous sequence with many closelyspaced frequencies allows the piezoelectric motor 20 to closely trackcurve 102 even when shifted. Each such sweep causes the piezoelectricmotor 20 to execute a well-defined step of the driven element 22 whichis related to the area under the curves 101 or 102. If the sweep can beexecuted repeatedly and sufficiently fast, the individual steps blendtogether into what appears to be a smooth motion of the driven element22 in the sense that monitoring means and criteria set forth by anapplication cannot tell that the motion is indeed composed of manyindividual and distinct steps. The speed of said smooth motion is givenby the product of the step size and the number of repetitions persecond. As discussed above, the criteria for determining whatconstitutes “smooth” motion will vary with the particular application,and the frequencies are repeatedly executed sufficiently fast to achievethe required smoothness of motion.

There is thus provided an open-loop control method that produces definedstep sizes of the driven element 22 at a steady rate independently ofparameter fluctuations of a piezoelectric motor 20.

In addition to executing a sweep or any other sequence, thepiezoelectric motor 20 can be further slowed down by periodically, ornon-periodically, turning it on and off, e.g., by interrupting theelectric driver signal 25 to the piezoelectric motor 20 atpre-determined times for pre-determined amounts of time. For a sweep, apreferable moment to interrupt the electric signal 25 is when the sweephas reached its end and before it recommences. If the first and the lastfrequency of a sweep both lie outside the range of operation, thepiezoelectric motor 20 has stopped anyway at this time. The signalinterruption therefore should not produce an audible sound. Theexecution of a sequence of frequencies may also be interrupted if, forexample, the driven element has reached a predetermined destination, orif the current sequence of frequencies does not provide the desiredpiezoelectric motor 20 performance. In the latter case, a sequence offrequencies may be modified or be replaced by a more appropriatesequence of frequencies using a feedback method as discussed later.

The rate of change of a frequency sweep, the rate of change given by thedurations of the associated waveforms, does not need to be constant.Indeed, if possible, it is advantageous to sweep slowly where it isknown or estimated that the piezoelectric motor 20 goes through atransition from not operational to operational in order to reduce oreliminate audible noise, which is typically generated when thepiezoelectric motor 20 is abruptly set in operation, or abruptly stopsoperation. It is thus preferable to slow the rate of sweep, or toincrease the duration of the associated waveform, so that as thetransition of the piezoelectric motor 20 from an operational tonon-operational mode does either not produce an audible sound or producea predetermined audible sound. This variation in the rate of sweep orvariation in the duration of the associated waveform can also be used tomeet other criteria at the desired transition point. One example wouldbe to produce a sound at a predetermined volume or amplitude, or togenerate a predetermined signal that may vary with the use to which themotor 20 is put.

A sequence of discrete frequencies is suitable for means of signalgeneration 23 that are digital where the signal waveforms arerectangular, or digital, as opposed to being sinusoidal. Digital signalgeneration 23 can be achieved, for example, with an appropriatelyprogrammed microcontroller, or with a pulse-width modulation (PWM) unit,which is often comprised in a microcontroller. Digital signal generatorsare limited by the fact that the time resolution of the generated signalis the product of a signal generator specific time constant and aninteger. Strictly periodic signals are therefore only possible atcertain discrete frequencies. This property of digital signal generationis explained referring to FIG. 5. In this figure, the time resolution ofa digital signal generator 23 is given by a constant ΔT. A firststrictly periodic signal is one that, for example, repeatedly is highfor a period of 4ΔT and low for an equal period as in digital signal 80.The base frequency of digital signal 80 is thus 1/(8ΔT). A secondstrictly periodic signal with base frequency 1/(10ΔT) is digital signal81, which has low and high periods of 5ΔT. A third strictly periodicsignal with base frequency 1/(9ΔT) is digital signal 82, which has lowperiods of 5ΔT and high periods of 4ΔT. The base frequency of a strictlyperiodic signal is thus 1/(NΔT), where N is a positive integer. This isa limiting factor in selecting sequences of frequencies and can furtherbe a limiting factor in achieving constant piezoelectric motor 20performance as can be understood from the previous discussion of FIG. 1and the differences in performance caused by the previously discussedsequences {f₁ . . . f₉} and {f₁ . . . f₅}. In other words, if thefrequency resolution of the digital signal generator is poor withrespect to the width of the range of operation, the resultingpiezoelectric motor 20 performance may be less robust with respect tochanges of piezoelectric motor 20 parameters such as temperature, etc.

Fast switching between adjacent frequencies, for example between 1/(NΔT)and 1/((N+1)ΔT), provides a method to operate a piezoelectric motor 20at frequencies that a digital signal generator cannot readily generatein a pure form. In this method, a sequence of frequencies {F₁, F₂, F₁,F₂ . . . } is composed of two adjacent frequencies F₁ and F₂ in repeatedpairs that the digital signal generator can readily generate in a pureform. It was previously discussed that if each of F₁ and F₂ is executedfor a relatively long duration, then the piezoelectric motor 20performance toggles between the performances corresponding tofrequencies F₁ and F₂. However, if the switching occurs fast, i.e., ifthe duration for which each of F₁ and F₂ is executed is smaller than thedecay time for piezoelectric motor 20 transients, then the piezoelectricmotor 20 performance is not given time to settle into eitherperformance, but instead the piezoelectric motor 20 can be viewed asbeing presented with a signal 25 that has a principal frequency contentF₃ that lies in between F₁ and F₂. The exact location of F₃ depends onthe ratio of the durations for which F_(1 and F) ₂ are individuallyexcited and may be determined with a standard mathematical tool known asFourier analysis. For example, if each of F₁ and F₂ is excited for thesame duration, representing a duration ratio of 1:1, then F₃ lies in themiddle between F₁ and F₂. In principle, any other frequencies F₃ can beapproximated sufficiently close using other duration ratios.Realistically however, the duration ratios are limited by the durationof the piezoelectric motor 20 transients and the time resolution of thedigital signal generator 23. Piezoelectric motor 20 are generally veryresponsive and can have transients that are as short as four or fivevibration periods.

For example, if the mechanical piezoelectric motor 20 transients occurwithin 4 oscillation periods, then the duration for which each of F₁ andF₂ is applied for should be less or equal than those 4 oscillationperiods. If the durations are selected to be convenient integermultiples of the vibration periods, then the duration ratios in thisexample are approximately 1:1, 1:2, 1:3, 1:4, 2:1, 2:3, 3:1, 3:2, 3:4,4:1, and 4:3, giving rise to an equal number of frequencies F₃ that liebetween F₁ and F₂. The said duration ratios are approximate in the sensethat in a preferred application of the method, the duration for which afrequency is applied is advantageously an integer multiple of the periodof that frequency. In particular, four periods of two adjacentfrequencies have nearly, but not exactly, the same duration. It isadvantageous to use said integer multiples of vibration periods to avoidsudden signal 25 jumps and possibly resulting jarring noises of thepiezoelectric motor 20, but durations do not need to be integermultiples. It can also be convenient to use durations that are integermultiples of one half of a period.

There is thus provided means for achieving usable operation from apiezoelectric motor 20, even if the optimal operational frequency ofthat piezoelectric motor 20 has changed, by using means of digitalsignal generation 23 to provide a sequence of frequencies to thepiezoelectric motor 20 at predetermined intervals within a frequencyrange sufficient to cause the piezoelectric motor 20 to operate lessthan optimally but reliably. The sequence of frequencies may containsub-sequences of frequencies that the digital signal generator 23 cangenerate in a pure form, but that are each of a duration that is shorterthan the typical duration of a piezoelectric motor 20 transient, for thepurpose of operating the piezoelectric motor 20 at frequencies that thedigital signal generator 23 cannot generate in a pure form.

The proposed control methods can be augmented and improved by anappropriate feedback mechanism by which any piezoelectric motor 20operation (optimal or not) is detected. Several different methods can beused. The fact that the piezoelectric motor 20 is operating, i.e., thatit is adequately moving a driven element 22, can in some instances bederived from an electric response 30 of the piezoelectric motor 20,e.g., from a phase shift between voltage and current, or from anincrease/decrease in current consumption, or from an increase/decreaseof voltage at the piezoelectric element. Further sources of feedbackinformation 27 are single or combinations of sensors 26 that detectmotion of the driven element directly, such as Hall sensors or lightbarriers, or also force sensors. A Hall sensor or a light barrier can beused to provide impulses every time the driven element has moved adefined distance and/or passed selected thresholds. Distance may bemeasured as length for a linearly moving driven element, or as angle forrotating driven elements such as wheels. Counting the number of impulsesduring a determined period of time can provide a measure of speed of thedriven element 22.

In another example, a piezoelectric motor 20 comprises an electricallyconductive resonator 28 that is in driving contact with an electricallyconductive driven element 22. Measuring the electric resistance betweenthe resonator 28 and the driven element 22 may provide the desiredfeedback 30 in a piezoelectric motor 20 where the resonatorintermittently lifts off partially or completely from the driven element22 during piezoelectric motor 20 operation. The electric feedback signal30 in these cases may be discrete due to complete liftoff or analog dueto partial liftoff and/or change in contact pressure. In a preferredembodiment of the invention, one of the resonator 28 or the drivenelement 22 is made of a semi-conductive material, such as plasticcontaining carbon particles or fibers. In this embodiment, an analogsignal representing electric resistance can be used to provide feedback30 as to the frequencies at which the piezoelectric motor 20 operatesand at which frequency the piezoelectric motor 20 operates in an optimalsense.

For a particular piezoelectric motor 20, analyzing the feedback signal30 and/or 27 from a single or combinations of sensors 26 at anexcitation frequency, and deriving a numeric performance criteriondescribing the piezoelectric motor 20 performance such as speed or forceat that frequency using appropriate electronics and algorithms in acontroller 23 constitutes a feedback method. If an electric signal 25comprising a single, slow, continuous frequency sweep is supplied to thepiezoelectric motor 20, then the performance criterion traces aperformance curve as a function of the momentary excitation frequency.

Typical curves that may be obtained are illustrated in FIG. 2. Curve 51is representative of a feedback method that provides continuousinformation about piezoelectric motor 20 performance. Curve 52 isrepresentative of a feedback method that has a minimal motion thresholdand/or is hysteretic and thus provides discontinuous information aboutpiezoelectric motor 20 performance. Curve 53 is representative of afeedback method that provides only information about the presence of amotion with at least a minimal piezoelectric motor 20 performance. If aperformance curve of the type of curve 51 or 52 is measured, an optimalfrequency of operation may be determined at the maximum of theperformance curve. If a performance curve of the type of curve 53 ismeasured, an optimal frequency of operation can only be estimated, forexample at the horizontal center of the rectangular-shaped portion ofcurve 53.

Piezoelectric motor 20 performance curves may be different forcontinuous sweeps from low frequencies to high frequencies and viceversa.

Curves such as curves 51-53 may provide, in part, a way or the means todetermine a performance curve 102. A curve 102 may be selected tocoincide with one of curves 51-53, or additional information, forexample from a feedback signal 30, may me incorporated to compute acurve 102. As previously discussed, an idealized performance curve 101may be derived from the curves 102 that have been obtained understandardized conditions for many piezoelectric motors 20.

Approximations to curves such as curves 51-53 are obtained by usingsequences of frequencies to generate the electric signal 25 supplied tothe piezoelectric motor 20 instead of the continuous frequency sweep.For example, a curve such as exemplary curve 54 may be obtained by usinga sequence of frequencies {f₆, f₇, f₈, f₉}, analyzing the feedbacksignal 27 at each of these frequencies, and plotting the resultingpiezoelectric motor 20 performances as dots connected by, for example,straight lines.

When operating a piezoelectric motor 20 with a sequence of frequenciesto cause the piezoelectric motor 20 to move a driven element, then thefeedback method provides information about the piezoelectric motor 20performance while each frequency of the sequence is executed. Aspreviously mentioned, piezoelectric motor 20 are extremely responsive(as opposed to, say, DC electromagnetic motors with high inertia). Theinformation that is obtained with a feedback method therefore tracks thetiming of the execution of the sequence of frequencies with a delay ofonly a few vibration periods, provided that the delay in the feedbackloop is sufficiently small. There is thus provided a method to execute asequence of frequencies covering at least the range of operation of apiezoelectric motor 20 to move a driven element, and to simultaneouslyuse a feedback method to identify the piezoelectric motor 20 behavior,which changes over time, and to repeat an appropriate sequenceadvantageously within less than approximately {fraction (1/20)} of asecond to make the resulting motion of the driven element appearsufficiently smooth to the human eye, or to repeat the sequence fasterif a smoother motion of the driven element 22 is required, or to repeatthe sequence slower if an appropriate performance and smoothness ofmotion may so be achieved. The feedback information 27 and/or 30 canfurther be used to modify the sequence of frequencies to more closelytrack the changing range of operation of the piezoelectric motor 20.

The feedback method can be used to intermittently adapt a sequence offrequencies to cause the piezoelectric motor 20 to move the drivenelement 22 with an improved performance even if the properties of thepiezoelectric motor 20 have changed due to temperature, aging, or otherreasons. The procedure is illustrated with reference to the examples inFIG. 3. FIG. 3 shows an hypothetical performance curve 102 that apiezoelectric motor 20 may have at a particular point in time. In afirst example of intermittently adapting a sequence of frequencies, afirst sequence of frequencies {f₁, f₂, f₃, f₄, f₅} gives rise towaveforms w₁-w₅, which are supplied repeatedly to the piezoelectricmotor 20 in the sequence shown. During execution of the first sequenceof frequencies, a feedback method may provide a performance reading as afunction of time such as the exemplary curve 61 a. From curve 61 a, thecurve 102 can be estimated by graphing the measured values as verticesover the current excitation frequency and connecting the vertices withstraight lines, such as done in curve 55 a. Note that the vertices ofcurve 55 a or other such curves that estimate a curve 102 do notnecessarily lie on the hypothetical curve 102 due to, in part,measurement errors, etc. From the curve 55 a, one can estimate that thefrequency f₁ is far from the range of operation, and that the curve 102has a maximum that is likely located in the vicinity of f₈. While theseexamples refer to graphing and show various graphical images, thegeneration of these graphs is mathematically based and thus the analysiscan be entirely executed by appropriate software using a computer orappropriate integrated circuits or other electronic systems. Thisapplies to the above, and following graphs which are used to illustratethe principles of the described motor control.

One may use this information to determine, for example, a secondsequence of frequencies {f₂, f₃, f₈, f₄, f₅}, which when executed wouldgive rise to a performance reading as a function of time similar tocurve 61 b, and in the process to an estimate to curve 102 in the formof curve 55 b. Clearly, the second sequence of frequencies causes thepiezoelectric motor 20 to move the driven element 22 with an improvedperformance since it has more frequencies within the range of operationand also a narrower bandwidth than the first sequence of frequencies,the bandwidth being defined as the difference between the largest andthe smallest of the frequencies of a sequence. In addition, since thesecond sequence of frequencies covers the range of operation of thepiezoelectric motor 20, the piezoelectric motor can be reliably operatedeven if the range of operation, and therefore the curve 102, shouldshift by a small amount.

In a second example of intermittently adapting a sequence offrequencies, the feedback method is assumed to generate pulses when thedriven element 22 has moved by a sufficient distance. If, for example, afirst sequence {f₁, f₂, f₃, f₄, f₅} is repeatedly executed as done inthe previous example, then a performance reading as a function of timesuch as curve 62 a may be obtained. Curve 62 a illustrates that impulsesare likely to occur in faster succession the closer the frequency atwhich the piezoelectric motor 20 is being excited is to the optimalfrequency. The density of the impulse distribution in curve 62 a canthen be used to graph a curve similar to curve 55 a. Based on curve 55a, a second sequence of frequencies may be selected to cause thepiezoelectric motor 20 to move the driven element 22 with a betterperformance and an adequate robustness towards piezoelectric motor 20parameter changes when the second sequence of frequencies is executed. Apossible second sequence of frequencies is {f₂, f₃, f₈, f₄, f₅}. Inanother method, only the first occurrence of an impulse is used todetermine a likely operational frequency. In the exemplary curve 62 a,the first impulse occurs while f₃ is being supplied to the piezoelectricmotor 20. Based on this information a second sequence of frequencies maybe selected such as {f₂, f₃, f₈, f₄, f₅}, which may cause a performancereading in function of time such as curve 62 b with an associatedestimate of curve 102 given by, for example, curve 55 b. The secondsequence of frequencies has a more narrow bandwidth than the firstsequence and causes the piezoelectric motor 20 to operate with a betterperformance while maintaining a certain robustness towards changes ofpiezoelectric motor 20 properties. This method is particularly useful inembodiments where it can be assumed that the piezoelectric motor 20 ismost likely to trigger a response of the sensor 26 when thepiezoelectric motor 20 causes the driven element 22 to move with aperformance that is close to optimal. In all of the above methods, thesecond sequence of frequencies can be further modified or replaced withsubsequent sequences of frequencies in order to track moving curves 102due to changing piezoelectric motor 20 parameters while maintaining acertain degree of robustness towards changing piezoelectric motor 20parameters by selecting sequences of frequencies that have bandwidthsthat include at least the range of operation.

If the resulting piezoelectric motor 20 performance is required to besufficiently constant in an averaged sense, it is not necessary that afeedback signal is either produced, or evaluated to generate a modifiedsequence of frequencies, every time a sequence of frequencies, e.g., asweep, is executed. In embodiments where a microcontroller is used, itcan be advantageous to employ the feedback routine relatively rarely inorder to free up resources, but sufficiently often to achieve anappropriate improvement of the piezoelectric motor 20 performance due tofeedback. In particular, an adaptation of a sequence of frequencies isnecessary if significant performance degradation is observed, forexample, if no sensor 26 impulse is measured for a predetermined amountof time. Sensor 26 impulses may be used to trigger interrupts in acontroller 23, such as in a microcontroller that is capable andconfigured to receive and evaluate interrupts, in order to enable thecontroller 23 to adapt the sequence of frequencies. Using an interruptmechanism can further help free up microcontroller resources. Aninterrupt can also be used to reset a watchdog timer whenever a sensor26 impulse is received. The watchdog timer could then automaticallytrigger a microcontroller interrupt if no sensor 26 impulse is measuredfor a predetermined amount of time thus enabling a microcontrollerprogram to select a new sequence of frequencies at that time to improvepiezoelectric motor 20 performance.

Further additions to the control methods include occasionally switchingbetween the first sequence of frequencies and any subsequent sequence inorder to use the first sequence again to determine the possibly changedrange of operation of the piezoelectric motor 20. Also there is thepossibility that the range of operation only partially overlaps with thebandwidth of a sequence of frequencies due to predictable and sometimesunpredictable changes of piezoelectric motor 20 properties, or due to anunwise choice of a sequence of frequencies, thus reducing piezoelectricmotor 20 performance. When this occurs, the first sequence offrequencies can be selected again, or a new sequence of frequencies canbe selected that has a broader bandwidth than the second sequence offrequencies. This should particularly be the case if no or aninsufficient feedback signal is obtained for a certain period of time,say, for the duration of a frequency sweep. Said bandwidth can besubsequently broadened even further if still no feedback signal isgenerated, presumably because the driven element 22 is being movedinsufficiently, until a feedback signal is observed, presumably becausethe driven element 22 is moving sufficiently again to cause a feedbacksignal.

The open-loop and feedback control methods discussed here areparticularly useful if the piezoelectric motor 20's range of operationis known only in vague terms. In a preferred embodiment of theinvention, this kind of feedback control is advantageously carried outwith a microcontroller. There are thus provided methods for identifyingoperational frequencies of piezoelectric motor 20 and for using one ormore of those operational frequencies to drive a piezoelectric motor 20in a manner suitable to achieving an acceptable performance.

The control systems and methods that are described here are particularlysuitable for controlling single-frequency piezoelectric motors 20. Thecontrol schemes can be used by themselves, in combination with eachother, or used in various combinations with other existing controlschemes. The piezoelectric motor 20 control is essentially open-loop butthe control methods allow the repeated and intermittent update of theessentially open-loop control by way of means of feedback. The controlmethods provide an amount of robustness towards predictable andunpredictable changes of piezoelectric motor 20 parameters, which inpart may also depend on the mechanical load encountered by the drivenelement 22. Predictable piezoelectric motor 20 parameter changes areforeseeable changes that are known at the time of the piezoelectricmotor 20 control design. Predictable parameter changes are changes thatcan reasonably be expected during intended use of the piezoelectricmotor 20 and driven element 22 and do not principally interfere with theapplication of the control methods disclosed herein. For example,changes in piezoelectric motor 20 temperature, ambient temperature,motor wear are predictable parameter changes, while motor breakage isnot.

Preferred embodiments may contain several piezoelectric motors 20 tomove a single driven element 22. One such multi-motor configuration isillustrated in the schematic diagram of FIG. 6 where three piezoelectricmotors 22 a, 22 b, 22 c are in simultaneous driving contact with asingle driven element 22. The piezoelectric motors 22 a, 22 b, 22 c maybe supplied with individual electric control signals 25 a, 25 b, 25 cfrom separate electric driver circuits 24 a, 24 b, 24 c, that arecontrolled from separate controllers 23 a, 23 b, 23 c. Alternatively,the piezoelectric motors 22 a, 22 b, 22 c may be supplied with the sameelectric control signal 25 from one electric driver circuit 24 and onecontroller 23 in which case signals 25 a, 25 b, 25 c are identical,drivers 24 a, 24 b, 24 c are one and the same, and controllers 23 a, 23b, 23 c are one and the same.

Alternatively, a single controller 23 may control the individual drivercircuits 24 a, 24 b, 24 c generating electric signals 25 a, 25 b, 25 c,in which case controllers 23 a, 23 b, 23 c are one and the same. Thepiezoelectric motors 22 a, 22 b, 22 c have individual feedback paths 30a, 30 b, 30 c. Controllers 23 a, 23 b, 23 c share the same feedback froma device 26 that detects motion of the driven element 22. In multi-motorapplications, an advantageous averaging of mechanical piezoelectricmotor 20 a, 20 b, 20 c output across all piezoelectric motors 20 a, 20b, 20 c that are engaged with the driven element 22 goes into effect. Amulti-motor configuration can also have two or more piezoelectric motors20 in driving contact with a single driven element 22. The individualpiezoelectric motors 20 can be of identical design and manufacture butmay also be different, which may further produce an advantageousaveraging effect where the strength and weaknesses of variouspiezoelectric motor 20 designs and manufactures are balanced.

The above description is given by way of example, and not limitation.Given the above disclosure, one skilled in the art could devisevariations that are within the scope and spirit of the invention,including various ways of arranging the piezoelectric motors 20 and ofselecting appropriate sequences of frequencies and of sweeping thesefrequencies. Further, the various features of this invention can be usedalone, or in varying combinations with each other and are not intendedto be limited to the specific combination described herein. Thus, theinvention is not to be limited by the illustrated embodiments but is tobe defined by the following claims when read in the broadest reasonablemanner to preserve the validity of the claims.

1. A method for controlling a piezoelectric motor, the piezoelectricmotor being configured to move a driven element when a sinusoidalelectric signal of a first frequency is supplied to the piezoelectricmotor with an amplitude that is sufficient to move a driven element apredetermined distance, comprising: selecting a predetermined firstsequence of frequencies, the first sequence of frequencies comprising atleast two mutually different frequencies; creating individual waveformscorresponding to each frequency of the first sequence of frequencies sothat each individual waveform has a predetermined finite duration andamplitude and is periodic with a period that is the inverse of thecorresponding frequency; concatenating the individual waveforms into asingle first electric signal and supplying said signal repeatedly to thepiezoelectric motor to move the driven element; and wherein the selectedfirst sequence of frequencies comprises a sufficient number offrequencies that are distributed to cause the piezoelectric motor tomove the driven element even when the properties of the piezoelectricmotor change within a predictable range.
 2. The method of claim 2,wherein the piezoelectric motor has a performance that, when averagedover the duration of the single first electric signal, said performancedoes not vary more than 30% when the properties of the piezoelectricmotor change within a predictable range.
 3. The method of claim 1,wherein the single first electric signal causes the piezoelectric motorto move the driven element with a varying performance, furthercomprising: monitoring the motion of the driven element; selecting thefirst sequence of frequencies to further comprise a sufficient number offrequencies that are distributed to cause the piezoelectric motor tomove the driven element so that the motion of the driven element ismaintained within a predetermined value as determined by the monitoringof the driven element and so that said monitoring cannot perceive thevarying performance caused by the first electric signal.
 4. The methodof claim 1, further comprising a motion detector that provides afeedback signal when the motion of the driven element passes at leastone selected threshold, the method further comprising: analyzing thefeedback signal to determine an estimated frequency at which thepiezoelectric motor can movie the driven element when a sinusoidalelectric signal of the estimated frequency is supplied to thepiezoelectric motor; selecting a second sequence of frequenciescomprising at least the estimated frequency and at least one otherfrequency that is different from the estimated frequency to generate asecond electric signal in accordance with the method of generating thefirst electric signal to cause the piezoelectric motor to move thedriven element with an average performance that is higher than theaverage performance of the piezoelectric motor was before the estimatedfrequency was determined; repeatedly analyzing the feedback signal andselecting a second sequence of frequencies as often as needed to achievea desired duration and distance of motion and a desired average motorperformance.
 5. The method of claim 4, wherein the second sequence offrequencies comprises at least one frequency that is smaller than theestimated frequency and at least one frequency that is larger than theestimated frequency.
 6. The method of claim 4, further comprising:analyzing the feedback signal to determine if the motion of the drivenelement has been less than a predetermined value for a predeterminedamount of time; modifying the second sequence of frequencies when thepredetermined amount of time has passed to have at least a differencebetween the largest and the smallest frequency of the sequence that islarger than the difference between the largest and the smallestfrequency of the unmodified second sequence; repeatedly analyzing thefeedback signal and modifying the second sequence until it is determinedthat the motion of the driven element is no longer less than thepredetermined value for that predetermined amount of time.
 7. The methodof claim 1, wherein the first sequence of frequencies is selected tocause the piezoelectric motor to move the driven element by a defineddistance even if the properties of the piezoelectric motor change due topredictable causes.
 8. The method of claim 7, further comprising thestep of supplying the first electric signal a predetermined number oftimes per second to cause the piezoelectric motor to move the drivenelement at a defined speed.
 9. The method of claim 1, wherein any twoconsecutive frequencies each produce a piezoelectric motor performancecomprising at least one of (the speed of the motor, the speed of thedriven element, the motion of the motor, the motion of the drivenelement, and power consumption of the motor), with a performancedifference between each of two said consecutive frequencies that is nomore than a predetermined value.
 10. The method of claim 1, wherein thefrequencies are selected to produce a predetermined audible sound thatis generated by the piezoelectric motor.
 11. A method for selecting asequence of frequencies for controlling a piezoelectric motor to move adriven element, the piezoelectric motor being configured to move thedriven element when a sinusoidal electric signal of a first frequency issupplied to the piezoelectric motor with an amplitude that is sufficientto move a driven element a predetermined distance, comprising: selectingat least two frequencies that are mutually different to form a sequenceof frequencies to cause the piezoelectric motor to move the drivenelement when individual waveforms corresponding to each frequency ofsaid sequence of frequencies are concatenated to form an electric signalthat is supplied repeatedly to the piezoelectric motor to move thedriven element, each of said individual waveforms having a predeterminedfinite duration and amplitude and being periodic with a period that isthe inverse of the corresponding frequency; selecting the at least twofrequencies to further cause the piezoelectric motor to move the drivenelement even if when the properties of the piezoelectric motor changewithin a predictable range.
 12. The method of claim 11, furthercomprising the step of selecting the at least two frequencies to causethe piezoelectric motor to move the driven element by a defineddistance.
 13. The method of claim 12, further comprising the step ofselecting the duration of each of said individual waveforms to cause thepiezoelectric motor to move the driven element with a defined speed. 14.The method of claim 11, wherein the at least two frequencies each causethe piezoelectric motor to move the driven element with a differentvarying performance, further comprising the step of monitoring themotion of the driven element; and selecting the sequence of the at leasttwo frequencies to further comprise a sufficient number of frequenciesthat are distributed to cause the piezoelectric motor to move the drivenelement so that the monitored motion of the driven element meetspredetermined criteria said monitoring of the motion of the drivenelement cannot perceive the varying performance.
 15. The method of claim11, wherein the piezoelectric motor further comprises a motion detectorproviding a feedback signal when the motion of the driven element passesat least one selected threshold, the method further comprising:analyzing the feedback signal to determine an estimated frequency atwhich the piezoelectric motor can move the driven element when asinusoidal electric signal of the estimated frequency is supplied to thepiezoelectric motor; selecting the at least two frequencies to comprisethe estimated frequency to cause the piezoelectric motor to move thedriven element with an average performance that is higher than an theaverage performance of the piezoelectric motor was before the estimatedfrequency was determined.
 16. The method of claim 11, wherein thepiezoelectric motor comprises a motion detector providing a feedbacksignal when the motion of the driven element passes at least oneselected threshold, the method further comprising: analyzing thefeedback signal to determine if the motion of the driven element hasbeen less than a predetermined movement for a predetermined amount oftime; amount of time has passed to have at least a difference betweenthe largest and the smallest frequency of the sequence that is largerthan the difference between the largest and the smallest frequency ofthe unmodified sequence.
 17. The method of claim 10, wherein theselected frequencies are inaudible.
 18. A piezoelectric system having apiezoelectric motor driving a driven element so as to move the drivenelement in response to an electric signal, the motor having at least afirst sinusoidal operating frequency at which the motor moves the drivenelement an amount that meets predetermined criteria, the motor anddriven element having a desired performance criteria when operated atthat first operating frequency, the system comprising: a plurality ofconcatenated individual waveforms to form an electric signal that isrepeatedly supplied to the piezoelectric motor to move the drivenelement, wherein each of said waveforms has a predetermined finiteduration and amplitude and further is periodic with a period that is theinverse of a predetermined non-zero frequency associated with thewaveform, at least two of the frequencies associated with the pluralityof concatenated individual waveforms being mutually different, and atleast one of the frequencies associated with the plurality ofconcatenated individual waveforms being sufficiently close to the firstoperating frequency to cause detectable motion of the driven element.19. The piezoelectric system of claim 18, wherein the frequenciesassociated with the plurality of concatenated individual waveforms arevaried in response to movement of at least one of the motor and thedriven element to produce an average performance of the motor and drivenelement for a time corresponding to the total duration of the pluralityof concatenated individual waveforms that is greater than an actualperformance of the motor and driven element for the same period of timebut when the actual performance is less than the desired performance.20. The piezoelectric system of claim 19, wherein the performancecriteria includes at least one of the speed of the driven element, theforce exerted by the motor on the driven element, and the power consumedby the motor.
 21. The piezoelectric system of claim 18, wherein thefrequencies associated with the plurality of concatenated individualwaveforms are a continually increasing series of frequencies.
 22. Thepiezoelectric system of claim 18, wherein the frequencies associatedwith the plurality of concatenated individual waveforms are acontinually decreasing series of frequencies.
 23. The piezoelectricsystem of claim 18, further comprising means for intermittently varyingthe frequencies associated with the plurality of concatenated individualwaveforms while maintaining at least one frequency which causessufficient motion of one of the motor or driven element to be detectedby a sensor.
 24. The piezoelectric system of claim 23, wherein the meansfor intermittently varying the frequencies associated with the pluralityof concatenated individual waveforms comprises a microcontroller. 25.The piezoelectric system of claim 24, further comprising a motiondetector to provide a signal when the motion of the driven elementpasses at least one selected threshold: wherein the microcontroller iscapable of receiving and evaluating an interrupt and is furtherconfigured to vary the frequencies associated with the plurality ofconcatenated individual waveforms when the signal triggers saidinterrupt of the microcontroller.
 26. The piezoelectric system of claim18, further comprising: a motion detector to provide a signal when themotion of the driven element passes at least one selected threshold; amicrocontroller configured to intermittently vary the frequenciesassociated with the plurality of concatenated individual waveforms; atimer to cause the microcontroller to maintain at least one frequencywhich causes sufficient motion of one of the motor or driven element tobe detected by a sensor when the signal provided by the motion detectorsignals that the motion of the driven element has been less than apredetermined value desired for a predetermined amount of time.